@melmoth after listening to the audio book and reading the story @Nearholmer mentioned, there are a number of mentions of the red light, two are below;
"He directed a most curious look towards the red light near the tunnel’s mouth, and looked all about it, as if something were missing from it, and then looked at me.
That light was part of his charge? Was it not?"
And
"I ran out again faster than I had run in (for I had a mortal abhorrence of the place upon me), and I looked all round the red light with my own red light, and I went up the iron ladder to the gallery atop of it, and I came down again, and ran back here."
This sounds more like a signal gantry to my untrained mind. Could it be that the signal is permanently 'on' until communication from the previous box is received and the line is then cleared, as is usual practice.
Thanks
Karl